1. Field of the Invention
The present invention relates to an electric potential switching circuit, in particular, to an electric potential switching circuit used in a flash memory.
2. Description of the Related Art
As widely known to those skilled in the art, various levels of electrical potential need to be supplied to a peripheral circuit of a flash memory such as a decoder in response to an operation mode of the memory. For example, it is necessary to supply a high electrical potential of about 10V to a word line decoder when the flash memory is set to a program mode in which a data writing (programming) is performed, and to supply an intermediate electrical potential of about 5V to the word line decoder when the flash memory is set to a verify mode in which a data verifying is performed.
In order to reduce an access time of the flash memory, it is required to switch the electrical potential immediately when the operation mode is switched. An Unexamined Patent Publication No. 2001-184879 discloses an apparatus for switching an electrical potential supplied to a row decoder from an electrical potential used for verifying to an electrical potential used for data writing at high speed. The publicly known apparatus has a writing system charge pump, a reading system charge pump and an electrical potential switching circuit. The writing system charge pump generates the electrical potential used for the data writing, and the reading system charge pump generates the electric potential used for the verifying and data reading. The electric potential switching circuit outputs to the row decoder one of the electrical potential generated by the writing system charge pump and the electrical potential generated by the reading system charge pump. According to the publicly known apparatus, while the verifying is performed, the electric potential output from the writing system charge pump is increased to a potential higher than a potential to be supplied to a memory cell in the data writing. Subsequently, when the data writing is started, the electric potential switching circuit outputs the electric potential output by the writing system charge pump to the row decoder. Thus, the electric potential output to the row decoder is raised rapidly.